(Refactor) Move welcome message to listener and add tests

[IonBazan]

This PR moves the logic responsible for sending welcome message to newly registered user into a listener. This makes it much easier to test the registration flow as it no longer depends on Bot and Chatroom classes and allows to test that particular logic separately instead.

Using event listeners instead of controllers/actions makes it easier to decouple the logic and allows to conditionally run it in the future if needed. It is also easy to mock it using Event::fake() in tests.

[HDVinnie]

@IonBazan great pull request. Fully agree with the changes here and really appreciate the test coverage which has been lacking unfortunately. Hopefully soon we can get a new core dev on board to manage tests and get our coverage 75% or greater. Thanks again.

[IonBazan]

Thanks for quickly merging the change. Glad to make a helpful contribution. I'll try to keep providing more similar improvements and hope to increase the coverage as the part of refactoring or fixing bugs too.